The invention relates to a bimetallic switch having a bimetal disc as the switching element.
Temperature controlled bimetallic switches are well known in the art. The bimetal supplies the force to open or close the contact system of the switch. These switches are used to prevent overheating or overcurrent conditions in a great number of electrical appliances, both large and small, such as household appliances, automobile components, and office automation equipment.
Various shapes of bimetallic elements are available, such as discs or cantilever strips. The thermal deflection of a strip element is usually easier to predict because formulas to predict deflection due to temperature are readily available. On the other hand, snap-acting bimetallic discs are also useful. These tend to change from a concave shape to a convex shape in response to changes in temperature.
There are three classifications to the type of bimetallic switches available in the art. These are: automatic reset, manual reset and one shot. Automatic resets have two distinct temperature points. A normally closed switch will open when exposed to heat at the higher of the two temperature points. When the switch cools, it will then automatically close at the lower temperature threshold. Manual reset switches, on the other hand, are typically closed, and have only an open threshold temperature. They require an outside force (such as, for example, a push button), to reset the bimetal disc after it has been opened. One shots will snap open only once and will not reset under normal conditions. They are actually automatic devices, except that the disc has been formed to have a low temperature point below xe2x88x92100xc2x0 Celsius. While one shots and manuals are typically used to protect a process, automatics are used on the other hand to control a process. There is, in addition, a hybrid version of an automatic switch, which is called a power down reset. The device includes a heat source which is activated when the contacts are opened because of increased temperature. This heat source produces enough heat to keep the bimetal above the low reset temperature threshold. In order for the device to reset, power to the switch must be removed and the temperature be low enough for the device to reset.
Bimetallic discs are said to xe2x80x9csnapxe2x80x9d when they change state from a crown up or dome shape to a crown down or dish shape when exposed to increased temperature. The time that the disc takes to snap is related, among other things, to the temperature change that it is exposed to.
One problem associated with the use of bimetallic switches is creep. The snapping over of the disc does not occur evenly across the snap time. There is in fact a transition state where the disc moves very slowly when compared to the total action time. This slow rate of movement is called xe2x80x9ccreepxe2x80x9d. Thus, when a formed snap acting bimetallic element gets close to its snap temperature (i.e. the threshold temperature at which the bimetallic element snaps in order to open or close the switch), it begins to deflect slightly. Depending on how the contact system is arranged, this small deflection or creep may lead to a contact gap prior to a snapping over of the bimetallic element. It is to be noted that the size of the arc is additionally related to electrical load and voltage as well as to other environmental factors. As a result, some thermostats may not arc because of the loads they switch.
Among examples of bimetallic switches of the prior art which may exhibit the problem of creep are those described in U.S. Pat. Nos. 4,862,133, 4,517,541, 4,424,506, 3,577,111, 3,067,306, 2,724,753 and 2,340,056.
Various designs for bimetallic switches have been proposed to resolve the problem of creep.
Japanese Patent Number 63-292539 discloses a bimetallic switch designed to prevent malfunction and vibrations during normal service. Here, as seen in the figures of that patent, the bimetallic disc 4 is supported by support pieces 7 having supporting surfaces 6, which are formed alongside the locus of the ends 8 of disc 4 under the service temperature thereof. Even though the disc deforms within the temperature range of its service temperature, the convex side of the central part 5 of bimetal 4 does not push the base 1 or movable contact piece 3, or separate therefrom, to a great extent. In this way, malfunction due to creep, and vibrations, may be prevented.
U.S. Pat. No. 5,121,095 uses an elaborate spring member independent of the contact arms to remove creep. The switch contacts are actuated via the bimetal to a spring member, to an insulated pin, and to a contact arm system.
Other current designs remove creep through an expensive measurement and custom part assembly process which involves the installation of a pin measured to very small tolerances, such as 0.001xe2x80x3, in each thermostat assembly. In such a case, a manufacturer must stock numerous pins to allow for the tolerance stack of the final assembly.
The above devices, which address the problem of creep, nevertheless do so at the cost of having to provide complicated and difficult to manufacture designs for bimetallic switches. Attention must be paid to the costs of manufacture including both labor and material costs.
It is therefore an object of the invention to provide a simple and cost effective bimetallic switch which allows for a creepless contact system. It is a further object to provide a bimetallic switch having a small footprint. It is a further object of the invention to provide a bimetallic switch where the disc is placed vertically, thus allowing for a relatively larger sized disc.
The above objects, and others to become apparent as the description progresses, is achieved by the provision of a bimetallic switch comprising a bimetallic element having an outer edge and being adapted to snap between a first mode and a second mode; a movable contact disposed on the bimetallic element; a first terminal in electrically conductively coupled to the movable contact; a fixed contact disposed adjacent the movable contact such that, when the switch is in a closed position, the fixed contact and the movable contact are in engagement with one another, and when the switch is in an open position, the fixed contact and the movable contact define an open contact gap therebetween; a second terminal electrically conductively coupled to the fixed contact; a step disposed adjacent the outer edge of the bimetallic element such that a clearance is defined therebetween when the bimetallic element is in its first mode, the clearance being positioned and dimensioned such that, when the outer edge of the bimetallic element deforms prior to a snapping thereof out of its first mode, the clearance isolates a deformation of the outer edge until the snapping of the bimetallic element. Advantageously, the bimetallic element is a bimetallic disc.
According to one embodiment of the invention, the switch further comprises a housing having inner walls defining an enclosure therein, wherein: the bimetallic element, the movable contact and the fixed contact are disposed within the enclosure; and the step is integral with the inner walls of the housing.
According to another embodiment, the bimetallic switch further comprises a cantilever, movable arm supporting the bimetallic element at one end thereof; and a stationary arm disposed adjacent the cantilever arm and supporting the fixed contact thereon.
Optionally, the bimetallic element is oriented such that a snapping direction thereof is perpendicular to a direction of current flow into and out of at least one of the first terminal and the second terminal. Preferably, the first terminal and the second terminal are substantially planar, and the bimetallic element is a bimetallic disc oriented such that a plane including its outer edge is orthogonal relative to a plane of the first terminal and the second terminal. According to a further embodiment, the step is configured such that, when the outer edge of the bimetallic element has deformed just prior to a snapping thereof, the outer edge rests upon an engaging surface of the step substantially without a clearance therebetween.
Preferably, the housing is parallelepiped shaped, the bimetallic switch further including a cover for closing the enclosure.
According to another embodiment, where the bimetallic switch is a disc, the step is dimensioned and shaped to extend in a plane that is substantially parallel to a plane including the outer edge of the bimetallic disc and which intersects the bimetallic disc at a point within a predetermined range of 65 to 95 percent of a radius thereof when the bimetallic disc is in its closed mode.
According to one embodiment, the first mode of the bimetallic element corresponds to a closed position of the switch, and the second mode of the bimetallic element corresponds to an open position of the switch. In such a case, the movable contact may be positioned with respect to the fixed contact for maintaining a minimum open contact gap therebetween thereby isolating creep during contact closure. Alternatively, the first mode may correspond to an open position of the switch, and the second mode to a closed position of the switch.